Singlet Oxygen as an Intermediate in the Catalytic Fading of Dye Mixtures

The catalytic fading of some yellow azo dyes in the presence of blue, violet or red anthraquinonoid dyes has been investigated. It can be shown that this phenomenon occurs only in the presence of oxygen and is suppressed by adding typical singlet oxygen quenchers. On substituting singlet oxygen producers like methylene blue for the anthraquinonoid dyes, the catalytic fading still takes place, indicating that a photo-oxygenation reaction according to the type II mechanism is responsible. By carrying out competitive photooxidation experiments in ethylcellulose films, relative rate constants for the reaction with singlet oxygen in a rigid material were determined.